Vyvanse length

Trying to figure out if my teen is on the right dose of Vyvanse or just needs something else. How long until you noticed a difference when taking Vyvanse? She has been on 20mg that seemed to work but then didn’t and she was emotional. Now at 30mg for almost 4 weeks. We see better focus, better with time management, happier. But the afternoon is a crash with emotions. Do we keep trying a bit more in the 30 or should we consider other options? I’ve been told Vyvanse can take longer than say something like adderall. She’s also on Prozac 10mg.